There have been a number of prior proposals of bubble-pump systems. The following should be noted: U.S. Pat. No. 2,707,593 issued May 3, 1955 to Woodcock specifies the use of a conduit of such small dimensions laterally that the liquid therein will be vaporized to form vapour bubbles which fill the conduit from side to side, said bubbles being preceded and followed by slugs of hot liquid. U.S. Pat. No. 4,160,444 issued to Hamilton discloses two upwardly extending upflow heat collector conduits, subject to solar heating. Actually the heat collector conduits operate only one at a time in a cyclic manner. U.S. Pat. No. 4,237,866 issued to Rush describes a solar heater in which the discharge of mixed vapour and liquid from the heated passages is directed onto a heat exchange element extending through a combined header and heat exchanger. In the Rush device, however, heat is delivered from a higher position to a lower position by means of an externally powered circulating pump, whereas in the present invention heat is delivered autogenously. Reference is also made to my prior Canadian Patent No. 1,146,431 issued May 17, 1983 which describes a relatively simple bubble pump system upon which the present invention is based. Reference is also made to my U.S. patent application Ser. No. 275,917 filed June 22, 1981, now U.S. Pat. No. 4,552,208 claiming priority of the aforesaid Canadian patent.